During the past three decades immunotherapy has become an important approach for treating human diseases and conditions through the use of regimens designed to modulate immune responses. This is particularly important in pathological conditions where the immune system becomes compromised. Studies conducted in disease models and clinical trials demonstrate that augmenting host defense mechanisms is useful in treatment and prophylaxis against microbial infections, immunodeficiencies, cancer, and autoimmune disorders (1–5). Immune enhancing protocols may also have utility for promoting wound healing. In the process of wound healing, macrophages exhibit a principal role by modulating cellular proliferation and new tissue formation/regeneration. They also function as phagocytes, debridement agents and produce growth factors that influence the angiogenesis stage of wound repair (6).
Historically, the first immunostimulants developed were bacterial products (lysates and crude fractions), attenuated microbes or heat-killed bacteria. These included agents such as bacille Calmette-Guerin (BCG), Corynebacterium parvum, and lipopolysaccharide (1, 2). Although these agents have had limited success due to toxicities and side-effects, many have been licensed by the USDA for immunomodulation in veterinary medicine (3). Other substances have been developed from various sources and include those of natural origin, those derived by chemical synthesis or those synthesized using recombinant technologies. Most immunostimulants of natural origin are high molecular weight polysaccharides, glycoproteins or complex peptides (1). For example, three fungal polysaccharides derived from Schizophyllum commune (schizophyllan), Lentinus edodes (lentinan) and Coriolus versicolor (krestin) are currently in clinical use in Japan as biological response modifiers (4). Another polysaccharide, acemannan (isolated from Aloe vera), is licensed by the United States Department of Agriculture for the treatment of fibrosarcoma in dogs and cats (7). There are a few small molecular weight immunostimulants derived from natural products such as the glycosphingolipid KRN-7000. A clinical trial using KRN-7000 as an immunostimulant for treatment of solid tumors is currently in progress (8). Several immunostimulants of synthetic origin also have been developed that include compounds like isoprinosine and muramyl peptides (2). Recently a number of other immunomodulators of endogenous origin have been developed using recombinant technologies that have gained FDA approval. These agents include colony-stimulating factors, interferons and recombinant proteins (5). However, these compounds often have short half-lives and it is difficult to determine optimal dosage and appropriate combinations.
Although current immunostimulants show promise, there is still a need to develop more potent agents and increase the arsenal of available drugs for immunotherapy. One source of chemically diverse compounds that can be used for drug discovery of immunostimulants is natural products. For centuries natural products have been exploited as therapeutically useful agents, many of which are in clinical use today. Interest in natural products as a means to drug discovery is based on their unparalleled molecular diversity and rich spectrum of biological activities (9).
Since ancient times, microalgae have been used as a nutrient-dense food source. Historical records indicate that microalgae such as Spirulina platensis was consumed by tribes around Lake Chad in Africa and by the Aztecs living near Lake Texcoco in Mexico (10). During the last several decades there has been increasing interest in the commercial production of food-grade microalgae for human consumption and as feed for livestock. Among the various microalgae that have been explored for their commercial potential Spirulina species, Chlorella species and Aphanizomenon flos-aquae (AFA) are three major types that have been successfully produced and are in widespread use.
Both anecdotal reports and recent studies on the consumption of food-grade microalgae have reported enhanced immune function in both animals and humans. Oral administration of Chlorella pyrenoidosa has been correlated with enhanced natural killer cell activity (11) and granulocyte-macrophage progenitor cells (12) in mice infected with Listeria monocytogenes. Dietary Spirulina platensis increases macrophage phagocytic activity in chickens (13) and Spirulina fusiformis exhibits chemopreventive effects in humans (14). Human consumption of AFA has been reported to produce changes in immune cell trafficking and enhanced immune surveillance (15). The active components for all these effects have not been conclusively established.
Various compounds have been isolated from the microalgae studied herein. A number of polysaccharides and glycoproteins from Chlorella and Spirulina species have been characterized for their antitumor, antiviral and immunostimulating activity. In contrast, no such compounds showing any biological activity have been isolated from AFA.
A number of polysaccharides have been identified from Chlorella species that possess biological activity. In U.S. Pat. No. 4,533,548 an acidic polysaccharide was isolated from Chlorella pyrenoidosa that exhibits antitumor and antiviral activity (16). The glycosyl composition for this polysaccharide was mostly rhamnose, with minor amounts of galactose, arabinose, glucose and glucuronic acid. Another polysaccharide, isolated from marine Chlorella minutissima, reported in U.S. Pat. No. 4,831,020, appears to have tumor growth-inhibiting effects. However, no molecular weight or glycosyl composition was reported (17).
In U.S. Pat. No. 4,786,496, the lipid fraction (glycolipid portion) of marine Chlorella species displayed antitumor properties (18). Several glycoproteins have also been isolated from Chlorella species. For example, U.S. Pat. No. 4,822,612 reported a 45,000 dalton glycoprotein that has anticancer effects (19). Various other glycoproteins (20–23) and glyceroglycolipids (24) that may have immunopotentiating and antitumor properties also have been reported in the scientific literature. None of these compounds are polysaccharides.
Several different types of polysaccharides that exhibit biological activity have been isolated from Spirulina species. For example, the sulfated polysaccharide calcium spirulan inhibits tumor invasion and metastasis (25). Calcium spirulan (molecular weight 74,600 daltons) is composed of rhamnose (52.3%), 3-O-methylrhamnose (32.5%), 2,3-di-O-methylrhamnose (4.4%), 3-O-methylxylose (4.8%), uronic acids (16.5%) and sulfate (26).
U.S. Pat. No. 5,585,365 discloses that an antiviral polysaccharide with a molecular weight between 250,000 and 300,000 daltons was isolated from Spirulina species using hot water extraction (27). This polysaccharide is composed of rhamnose, glucose, fructose, ribose, galactose, xylose, mannose, glucuronic acid and galacturonic acid. A number of other low molecular weight polysaccharides that range between 12,600 and 60,000 daltons recently have been isolated from Spirulina species (28–30).
One way to determine immunostimulatory activity is to use a biological assay involving macrophages. Monocytes/macrophages are found in practically every tissue of the body where they are critical in coordinating immune responses and numerous biological processes (31). They play a major role in phagocytosis, immune surveillance, wound healing, killing of microbes and tumor cells, and antigen presentation to T lymphocytes (32). In cancer, macrophages mediate tumor cytotoxicity functions through the production of cytokines and other immune factors (33). In order for macrophages to play a major role in adaptive and innate immunity they must respond effectively to environmental agents by first becoming activated (34). Macrophage activation is mediated by proinflammatory transcription factors such as nuclear factor kappa B (NF-kappa B). Such transcription factors then control and modulate the activation/repression of an array of genes that mediate a variety of immune responses.
In unstimulated macrophages, NF-kappa B exists as inactive heterodimers sequestered by inhibitory-kappa B (I-kappa B) proteins within the cytosol. Agents that cause I-kappa B proteins to dissociate and degrade allow for the translocation of NF-kappa B dimers to the nucleus where they can activate transcription of downstream genes (35). Target genes regulated by NF-kappa B include proinflammatory cytokines, chemokines, inflammatory enzymes, adhesion molecules and receptors (36).
In this invention a transcription factor based assay for NF-kappa B in human monocytes was used to guide extraction, isolation, characterization and development of immunostimulatory polysaccharide preparations from food-grade microalgae. The polysaccharides of the present invention are both water soluble and soluble in aqueous ethanol solution unlike almost all other polysaccharides now available.